The mounting box is the part of a switch onto which the faceplate is screwed. They can either be surface-mounted on the wall or flush-mounted.
The size of the mounting box will correspond to the number of switches on the faceplate: one up to three-gang switches will only require a single mounting box, but if you are fitting a four-gang or six-gang switch a double mounting box will be needed.
Surface-mounted switches
Mounting boxes for surface-mounted switches can be metal or plastic. If using a metal mounting box there will be an earth terminal for connecting the earth core from the switch cable. Grommets need to be inserted into the cable holes to protect the cable. The depth of a surface-mounting box ranges from 16mm (5/8 in) to 41mm (1 ½ in).
Flush-mounted switches
Mounting boxes designed to be flush-mounted will be made of metal and have a depth of 25mm (1in). This type of mounting-box will require the earth core from the switch cable to be connected to the earth terminal inside the mounting box.
Mounting boxes for stud partition walls
When fitting a flush mounted switch to a stud partition wall you will need a box made specifically for this purpose. The boxes are made of plastic and have spring-loaded lugs to the sides, which hold the box firmly in place when it has been inserted into the hole cut in the plasterboard. Once again there are single and double-mounting boxes available.
Mounting boxes for architrave switches
Fitting an architrave switch requires an architrave-mounting box. Whether installing a one or two-gang switch you will have the choice of fitting a surface or a flush-mounted box. The flush-mounted architrave box is made of metal and 27mm (1in) deep. The surface-mounted architrave box is made from plastic and is 16mm -18mm (about ¾ in) deep.